1. Technical Field
This invention relates generally to apparatus for shielding against radiation and in particular to radiation shields for subsurface applications.
2. Description of Related Art
The characteristics of subsurface geological formations are of significant interest in the exploration for, production and monitoring of subsurface water, oil and gas. To that end, a variety of techniques have been developed to measure subsurface formation characteristics and evaluate the characteristics to determine petrophysical properties of interest such as fractional volume of pore space (“porosity”), the mineral composition of the subsurface formations and the fractional amount of such pore spaces that is occupied by particular connate fluids, such as oil, gas and water. These techniques typically include the subsurface deployment of tools or instruments equipped with sources adapted to emit energy into the formations (such deployment usually being through a borehole traversing the formations). The emitted energy interacts with the surrounding formations to produce various phenomena that are detected and measured by one or more sensors on the instrument. By processing the detected signal data, a profile or “log” of the subsurface formation characteristics is obtained.
A variety of measurement (“well logging”) techniques have been developed to evaluate subsurface formations some of which include emitting neutrons into the formations and evaluating the results of neutron interactions with formation nuclei. Neutrons have no electric charge and their mass is similar to that of a proton. Their mass in particular makes neutrons suitable for subsurface logging applications in which hydrogen-bearing fluids are present in the subsurface formations. In the formations, neutrons interact with hydrogen nuclei in the formations by losing energy and they react with other matter in the formations in a variety of ways. The characteristics of some of these interactions can be used to determine the formation properties.
Various types of radiation sources have been used in subsurface logging systems. For example, neutrons or gamma rays may be generated simply through the use of radioactive isotopes (which naturally decay over time), an x-ray source may be used or neutrons may be generated in a device utilizing a nuclear reaction to generate neutrons on demand. U.S. Pat. Nos. 3,255,353, 4,596,926, 4,698,501, 4,705,944, 4,810,459, 4,829,176, 4,879,463, 4,904,865, and 5,012,091 describe logging instruments equipped with active radiation sources and appropriate sensors. For neutron logging, isotope sources have the advantage of being virtually indestructible. Isotope sources have no electronic parts, and so can be relied upon to always produce neutrons (zero downtime). However, this is also a disadvantage of the isotopic source. Because the emission of neutrons cannot be controlled, strict radioactive safety procedures must be followed when handling such sources, and the logging instrument containing the source after the source is installed therein. This disadvantage prompted the development of electronic neutron sources.
High-energy neutrons may be generated through the controlled collision of energized particles by using a nuclear fusion reaction in the above described sources. Such a system is commonly referred to as a neutron generator. The generation of neutrons on demand by the use of energetic particle beams allows the construction of a neutron source which emits neutrons in bursts of well-determined duration and time sequences. One such pulsed neutron generator is described in U.S. Pat. No. 3,461,291. The neutron generator described in the '291 patent uses an accelerator tube in which charged particles, such as deuterium ions, are accelerated through an electric-static potential and collide with a target element such as tritium. The reaction between the deuterium ions with the tritium target produces almost monoenergetic neutrons at an energy level of about 14 MeV. In most applications the neutrons are not emitted continuously but in short bursts of well-defined durations and in repetitive sequences. When using such a pulsed neutron generator, the formation surrounding the instrument is subjected to repeated, discrete “bursts” of neutrons. U.S. Pat. Nos. 4,501,964, 4,883,956, 4,926,044, 4,937,446, 4,972,082, 5,434,408, 5,105,080, 5,235,185, 5,539,225, and 5,608,215 describe logging instruments equipped with such on-demand neutron generators.
In practice, the borehole and surrounding formation are irradiated with neutrons, and the various interactions of the neutrons with constituent nuclei are measured. The logging instruments are equipped with one or more sensors or detectors that record numbers of neutrons, particularly epithermal energy and thermal energy, as well as gamma rays which are emitted as a result of the interaction of the neutrons with the subsurface formations and the fluids in the borehole itself. The gamma rays may include inelastic gamma rays which are a consequence of high-energy collisions of the neutrons with atomic nuclei in the earth formations, as well as capture gamma rays emitted when low energy (thermal) neutrons are captured by susceptible atomic nuclei in the formations. Various gamma ray logging techniques and tools are described, for example, in U.S. Pat. Nos. 4,390,783, 4,507,554, 5,021,653, 5,081,351, 5,097,123, 5,237,594 and 5,521,378.
Properties of the formations which may be determined as a result of measuring neutron and gamma ray phenomena include formation density, fractional volume of void or pore space in the formation (porosity), carbon/oxygen (C/O) ratios, formation lithology, and neutron capture cross section (Sigma), among other measurements. Properties which may be determined by spectral analysis of the gamma rays include concentrations of various chemical elements, for example. Properties of fluids in the wellbore may also be determined from various neutron and gamma ray measurements.
Nuclear measurements are also applied in nuclear spectroscopy techniques to obtain qualitative and quantitative information related to subsurface fluid movement. U.S. Pat. No. 5,219,518 describes an instrument equipped with a neutron source and sensors adapted to measure water flow through nuclear oxygen activation. Alternative techniques for subsurface fluid measurements include the use of radioactive markers or tracers to identify flow path between formations or wells. U.S. Pat. Nos. 5,049,743, 5,182,051, 5,243,190, and 5,929,437 describe the use of elements that can be made radioactive by bombardment with neutrons so their location can be determined by nuclear logging. Logging tools equipped with gamma ray detectors are particularly suited to distinguish and determine the location of trace materials.
The nuclear phenomena detected with these instruments are representative of interactions not only with the formation nuclei, but also with the instrument and the borehole. In order to penetrate the formation, the fast neutrons must pass through the tool housing, the fluid in the borehole and casing in some applications before entering the formation. The resulting non-formation contributions to the measured radiations significantly complicate the analysis of the formation characteristics. The problem is all the more complex since the sensitivity of the detector(s) to the radiations coming from the borehole, instrument and the formation, is a function of many parameters, such as, to name a few, lithology, porosity, tool position in the borehole, borehole size, casing size/weight/eccentricity, cement quality, detector housings, or borehole fluid composition. Thus, it is important to take into account respective contributions of the non-formation elements.
For tools that generate and/or detect neutrons and gamma ray radiation, neutron shielding provides a means to moderate interactions of neutrons and components of both the tool itself and the immediately surrounding environment. In well logging applications, detecting gamma rays emitted from neutron interactions is particularly difficult due to the presence of the housing used to protect the gamma ray detector inside from pressure and abrasion. Neutrons interact with these housings to emit gamma rays which can be in the energy range of interest of neutrons interacting with the formations. Conventional techniques to shield against such neutron interaction include the use of shields disposed on the instrument. U.S. Pat. Nos. 3,947,683, 4,492,864, 4,220,851, 4,020,342, 4,390,783, 4,661,701, 5,081,351 and 7,148,471 describe the use of radiation shielding in well logging tools.
Neutron shielding known in the art typically includes homogeneous materials consisting of a rubber matrix with neutron absorbing (e.g., boron-10 containing) particle fillers. The filler particles are selected so as to absorb neutrons and emit capture gamma rays that are outside the energy range of gamma rays of interest resulting from neutron interaction with the formations. The rubber matrix holds the boron-10 containing particles together providing structural support. The amount (volume or mass) of such particle fillers in a homogenous shielding material is limited by the structural requirements of the shield to resist the borehole environment and to resist abrasion and mechanical damage. A need remains for improved radiation shielding structures for well logging tools.